Fibrous reinforcement is a well known method for improving the physical properties of cement and concrete structures. Asbestos fiber-reinforced cements have been widely employed as building materials. Asbestos-reinforced cements and concretes have long been used for the production of pipes, corrugated boards, roofing slates, machine foundations, storage tank walls, reactors, aircraft runways, roads, pilings and many other high strength articles. The type of asbestos which is satisfactory as fibrous reinforcement for cement is available in limited quantities. It is probable that the deposits of such workable asbestos will be exhausted relatively soon. Further, asbestos is now known to have carcinogenic effects. The carcinogenic properties of asbestos has lead to governmental regulations controlling and limiting its use.
Of the fibers currently used as an asbestos fiber replacement for cement reinforcement, polyacrylonitrile and polyvinyl alcohol fibers are preferred because they combine high fiber strength with good adhesion to a cement matrix. Unfortunately, both are expensive materials and significantly increase the cost of producing fiber reinforced cement structures.
A variety of other less-expensive materials have been considered for production of cement reinforcement fibers to replace asbestos-fibers. Steel fibers have been tried and found inadequate because they suffer from chemical attack by the alkaline cement environment. Various polymeric fibers have also been found to be inadequate. Glass and polyester fibers degrade due to the alkaline environment of the cement matrix. Nylon and cellulose fibers have been found to be too water sensitive to be successfully used.
Ideally, polyolefin fibers could be employed as an asbestos replacement for reinforced cement. Polyolefin fibers possess good inherent properties, such as alkaline resistance, good stiffness and tensile strengths and are relatively inexpensive. Unfortunately, a major obstacle to the use of polyolefins as a cement reinforcing fiber material is their inherent lack of affinity towards an alkaline mineral matrix. Further, polyolefin fibers are less dense than a cement slurry; that low density in combination with polyolefin's lack of affinity for a cement matrix allows polyolefin fibers to float to the slurry surface. Such poor dispersibility of polyolefin fibers results in poor reinforcement of a finished cementitious article.
Extensive efforts have been devoted to preparing polyolefin fibers, particularly polypropylene fibers, in a form which permits them to be successfully used as a replacement for asbestos fibers for reinforcement of cement and concrete structures.
Methods for fabricating polypropylene fibers in a way as to provide physical anchoring sites along the fibers for mechanical attachment of the cement matrix have been tried, as illustrated by U.S. Pat. Nos. 4,261,754; 4,414,030 and 4,477,522. European Patent Application No. 0 026 581 discloses that such fibers may be prepared by fibrillation of a polyolefin film which has been pretreated with a coupling agent such as an acid anhydride or methacryloxypropyltrimethoxysilane. Other methods involve the addition of certain thickening and clay additives to the cement slurry to hold the polypropylene reinforcing fibers in dispersion until the slurry sets, as illustrated by U.S. Pat. Nos. 4,363,666 and 4,428,775. Still another method to improve flocculation of polypropylene fibers when mixing with cement is to treat the cement-fiber mix with a water-soluble or emulsifiable polymer and a polyvalent salt such as Al.sub.2 (SO.sub.4).sub.3, as illustrated by U.S. Pat. No. 4,339,273.
Other procedures designed to render polypropylene fibers suitable as a cement reinforcing material involve the physical modification of the fiber surface by various means. To produce cement adherent fibers U.K. Patent Application No. 2,030,891 teaches a method for embedding inorganic powers in the surface of fibrillated polypropylene. Japanese Patent Publication No. 60 060 960 applies a fine aggregate to the fiber surface by means of a radiation hardenably epoxy binder to produce a cement adherent polypropylene reinforcing fiber.
Still other methods chemically treat the surface of polyolefin fibers to render the fiber surface more adherent to cement. Such methods include treatment of the polyolefin fibers with an aqueous dispersion of colloidal alumina or silica in conjunction with a chlorinated polypropylene, as taught by Japanese Patent Publication No. 7319849; a non-ionic or cationic polymer agglutinating agent such as a polyethylene oxide or poly(alkylaminoacrylate) as shown by Japanese Patent Publication No. 60 081 052; or a solution of an alkali or alkaline earth metal (bi) carbonate as disclosed in Belgium Patent No. 899,810.
Yet others have suggested chemical modifications of the base polyolefin from which the fibers are produced. Hence, U.K. Patent Application No. 2,021,552A states that an inorganic or organic acid group should be incorporated in the base polyolefin, either by copolymerization of acid monomers or by grafting acid groups to a prepared polyolefin, in order to improve the adherence to cement of fibers made of modified polyolefin. Polypropylene grafted with maleic anhydride is illustrated as an example of an improved material for producing cement reinforcing fibers. Japanese Patent Publication No. 49 036 748 also illustrates the use of maleated polypropylene as a material for producing cement reinforcing fibers. Another approach, as discussed in German Offenlegungsschrift DE 3341 462 A1, treats the polyolefin under elevated temperature and pressure with a solution of silane or SiCl.sub.4 together with silicic acid or a metal silicate and thereafter precipitates the reaction product as fibrils by lowering the temperature. The fibrils so produced are useful as asbestos fibers substitutes for reinforced cement structures.
Others have suggested that the surface of polyolefin fibers be catalytically reacted with reactive organic or inorganic Si (IV) compounds to render them adhesive to cement. Among the silanes disclosed as suitable for a method of this type are silicic acid anhydrides, organic halosilances, and silicate esters as discussed in European Patent Application No. 0 051 256 and German Offenlegungsschrift DE No. 32 10693 A1.
To date, polyolefin fibers have been made compatible as reinforcement fiber for cement only by incorporating additional compatibilizing agents into the cement slurry into which the fibers are admixed, by the special fabrication of the fibers to provide physical anchoring sites therein for mechanically anchoring to the cement matrix, by the chemical modification of the base polyolefin from which the fibers are then produced, or by the chemical modification of the fiber surfaces by costly and time consuming chemical reactions.
Ideally an agent could be found which when admixed in small quantities with a polyolefin stock material would allow the direct production of cement adherent fibers therefrom by conventional fiber production techniques. To date, no such additive has been disclosed by the art.